normal faulting in the path© geothermal area, central mexico

Normal faulting in the Pathé geothermal area, Central Mexico

Miguel Carrillo MartínezInstituto de Geología, UNAM, México, D.F., MEXICO

Received: January 16, 1997; accepted: February 18, 1998.

RESUMENEl campo geotérmico de Pathé está localizado en una secuencia volcánica del Mioceno Tardío al Plioceno en una región de

fallamiento normal activo. Estas fallas se agrupan en dos tendencias de dirección aproximada NS y cercana a la EW. El grupo EWes la extensión meridional de la semifosa tectónica de Aljibes cuya falla maestra es curva con un desplazamiento vertical compuestode 480 m.

El desplazamiento somero es hasta de 70 m, y el fallamiento EW está generalmente localizado a lo largo de un área semi-elíptica de 34 km constituida por una meseta basáltica y conos cineríticos. El fallamiento NS define la fosa tectónica de SanFrancisco, con desplazamientos verticales de alrededor de 30 m, incrementándose hasta 180 m en el subsuelo. Estas rocas Neógenasestán intrusionadas por numerosos diques de dirección EW y troncos de rumbo NS.

Producción de vapor generalmente ocurre en la zona de intersección de la parte oriental de la fosa tectónica de San Franciscocon el fallamiento EW. Fuentes hidrotermales activas están localizadas sobre una de las fallas de rumbo EW. Alteracioneshidrotermales a lo largo de segmentos de fallas y fracturas sugieren actividad hidrotermal antigua.

PALABRAS CLAVE: Campo geotérmico Pathé, geología estructural.

ABSTRACTThe Pathé geothermal area is located in Late Miocene to Pliocene volcanic rocks in a region of active normal faulting. These

faults cluster in sets striking near E-W and close to N-S. The east-west fault set is the southern extension of the Aljibes half-grabenwhose master fault is curved and has a composite vertical offset of 480 m.

Near-surface vertical displacement is up to 70 m, while E-W faulting is generally located along a 34 km long semielliptic areaof plateau basalts and cinder cones. The N-S faulting defines the San Francisco graben, with vertical offsets of around 30 m,increasing up to 180 m below the surface. Neogene rocks are intruded by numerous roughly east-west trending dikes and north-south trending stocks. Steam production occurs at the intersection of the eastern San Francisco graben with east-west faulting. Hotspring activity is located on an E-W trending fault. Hydrothermal alterations along fault and fracture segments suggest earliergeothermal activity.

KEY WORDS: Pathé geothermal field, structural geology.

INTRODUCTION

The study area for this paper (Figure 1) lies near thenorthern edge of the Mexican Volcanic Belt, between thestates of Hidalgo and Querétaro (Segerstrom, 1962; Suter etal., 1995). The area consists mainly of Miocene to Pliocenevolcanic rocks with fault-bounded blocks and a regionalalluvial valley. The strike of faulting is approximately E-Wand N-S, from Landsat imagery (see Figure 2 of Suter et al.,1995).

We map the area with emphasis on near-surfacestructure, because it appears that the faulting governs presentand past geothermal activity (Nichols, 1970).

The lithostratigraphy consists of lower-Cretaceouscarbonate platform rocks and late-Cretaceous marly and shalyrocks covered by late Miocene to lower Pliocene continentalvolcanic rocks. The latter include a 100 to 400 m thick basalt

with 40Ar and 39Ar ages of 7.7 ± 0.1 M.a. and 7.1 ± 0.5 M.a.(Suter et al., 1995), sandwiched between andesitic andrhyolitic lavas and tuffs (Figure 2, and 3). At three locations(Figure 3) hypabyssal dioritic, basic and porphyritic graniticigneous intrusions have been identified (Figure 4). Sanidinefrom a sample of the northern hill of a rhyolite dome locatedclose to the main steam production yielded K/Ar dates of6.6±0.1 m.y. and 6.7±0.1 m.y. (Nichols, 1970). Between Bajhiand El Salto, a swarm of aphanitic andesitic dikes and sillstrend roughly E-W.

STRUCTURAL GEOLOGY

Most of the regional deformation in the study areaconsists of high-angle normal faults and small but widespreadfractures filled by veins and dikes (Figure 3). Six shieldvolcanoes of the “basaltic unit” are aligned NNE. One set offaults strikes near E-W and another close to N-S (Figures 1and 3). There is little regional faulting south of the area. Just

Geofísica Internacional (1998), Vol. 37, Num. 2, pp. 103-111

103

M. Carrillo Martínez

Fig

. 1. L

ocat

ion

map

of t

he a

rea

(see

inse

t).

104

Faulting in the Pathé area, Central Mexico.

north and northeast there are five regional E-W normal faultsin the Aljibes half-graben (Suter et al., 1995). Farther to thenorth, in the Sierra Madre Oriental fold-thrust belt, regionalnormal faulting is rare (Segerstrom, 1961, 1962; Suter, 1987a;Carrillo, 1989). To the southwest, regional lineaments strikingroughly E-W are observed from satellite imagery. Towardthe east and southeast, normal faults with a few meters ofthrow exist in Huizache, La Cruz, and elsewhere beyond thearea. Approximately 25 km to the east, the western fault-mapped segments of the seismically active Mezquital grabenare found (Suter et al., 1996).

East trending normal faults. In the northern region ofthe Aljibes half-graben the normal faults display a consistentdown-to-the-south sense of slip (Suter et al., 1995). In thestudied area, however, E-W faulting is both down to the northor to the south. Sharp discontinuous escarpments are 1 to 30m high. The four main faults are labeled F, G, H and I (Figures1 and 3).They represent the southern continuation of the A-E faults of the Aljibes half-graben. Fault A (beyond the studyarea) is curved and is associated with a major rollover fold(Suter et al., 1995).

Fault E. At the San Juan River near its junction with

the Tecozautla River, fault E strikes roughly E-W and dips75° to the south; it displaces the base of an upper basalticflow by 30 m vertically (Figure 5). This fault extends to thewest at least 14 km with a throw of 15 m at the Pathé cliffand across the hydrothermal sources at Taxhido, which arepart of a total yield of 600 l/s (R. Riva Palacio, personalcommunication). On the western extension Nichols (1970)reported a major horst near the low-water ford of the SanJuan River which separates the main steam-producing zoneto the south from a smaller graben to the north. The top ofthe upper rhyolite unit is displaced 20 m downward in thenorthern graben and 180 m in the southern faulted block nearthe NS trending Pathé fault. The steam-producing master faultof the graben extends to the west beyond its intersection withthe Las Rosas fault. At this locality a small horst is definedby a northern fault striking N70ºE and dipping 70º-80º to thenorth, and by the parallel E fault dipping 70º-80º to the south.This horst shows 30 m of throw as estimated from the contactbetween a thick-bedded rhyolitic tuff and the overlyingandesite lavas.

Fault F. West of Banzhá a normal fault strikes N80ºEand dips 70º-80º to the north. It offsets vertically the base ofa basaltic flow by about 15 m. It extends more than 17 km to

Fig. 2. Generalized lithostratigraphic section of the Pathé geothermal field and surrounding area. The units were introduced by Nichols(1970).

105


Fig

. 3. S

truc

tura

l map

of t

he s

tudy

are

a an

d st

ruct

ural

cro

ss s

ectio

n. N

-S a

nd W

-E r

efer

to th

e tr

ace

of th

e cr

oss

sect

ion.

A,

B a

re th

e no

rmal

faul

ts b

orde

ring

the

Alji

bes

half-

grab

en.

106


Fig. 4. Mafic dome outcropping 2 km NE ofTecozautla. The host rocks are beds of the

“pumitic unit”.

Fig. 5. Fault E across the Tziquia cliff looking tothe west. The section pictured corresponds to the

“basaltic unit”.

Fig. 6. Looking east at fault G, 2 to 3 km northeastof Tecozautla River. A dark-gray tuff representsthe marker horizon to estimate a throw of about

30 m.

107


the west. In the San Juan River, near its intersection with theLas Rosas fault, a segment of the F fault shows a verticalescarpment 30 m high along the San Juan River. Theescarpment height diminishes toward the west where itintersects the Las Rosas fault.

Fault G. In the cliff located west of Banzha an E-Wstriking fault dipping 65º-75º to the north displaces by 7 mthe base of a basaltic bed which caps the pyroclastic beds ofthe “pumice tuff unit”. The fault branches near the top of thehill. To the west the throw of this fault increases and reachesat least 25 m of vertical displacement in the valley of theTecozautla River. Farther to the west, at the San FranciscoRiver, this fault displaces vertically the base of a basalticflow by 30 m.

Fault H. About 7 km to the ENE of Tecozautla the Hfault crops out. At this locality the fault strikes N75ºW anddips northward 75°-80°. A black ignimbrite bed is verticallydisplaced by 30 m. The fault juxtaposes different beds of thepumitic unit (Figure 6). A scarp 3 m high shows well-definedstriae. At Arroyo Bajhi, along the eastern extrapolation ofthis fault, a normal fault dips 70° toward the south. The faultoffsets semiconsolidated alluvial fan deposits againstpyroclastic rocks of the interstratified volcanic unit (Figure7). Farther east of this outcrop there is a normal fault strikingN80ºW and dipping 70º-80º to the north. At this locality thefault juxtaposes the limestones of the El Doctor Formationagainst rocks of the interstratified unit and against a narrowoutcrop of the Soyatal Formation along a straight contact;well-defined striae are found in many parts of the fault. Aflat of the Sierra Madre Oriental fold-thrust belt is displacedvertically about 70 m (Figure 3).

Fault I. Fault I is clearly exposed a few kilometers tothe ENE of the town of Tecozautla. The fault juxtaposes thebasalt unit against the pumitic unit (Figure 8). It strikesN80ºW and dips 85° to the north showing gouge, tectonicbreccia and hydrothermal alteration. Intense fracturingsubparallel to the fault plane exists in both blocks. Theseclose-spaced fractures are filled with hydrothermal alterationminerals and clay deposits (illite and kaolinite), some ofrecent weathering. In the basaltic beds they are over 1 mwide. The minimum throw is 30 m.

San Francisco graben (north-south faults). These faultsdevelop in the western region of the study area strikingbetween N10ºW ± 10º and ≈ N30ºE. They corrrelate with ~N-S normal faults in northern Mexico (Stewart, 1978; Suter,1987b; Henry and Aranda, 1992; Aguirre and Mc Dowell,1993). The structure of this region is dominated by the Pathéand the Las Rosas faults of kilometric length, which define agraben here called San Francisco. The eastern Pathé fault(Nichols, 1970) crops out discontinuously along its trace andfeatures a discontinuous scarp. In the transverse valley,faulting is manifested by the abrupt truncation of bedding,

tectonic breccia and, in places, slickensides. The mappedlength of the trace of this fault is 14 km striking betweenN10ºE to N10ºW and dipping 70º-85º to the west at thesurface. At the San Juan River the base of the youngestoutcropping basalt flow is vertically displaced about 70 m.

La Mesa fault. West of the Pathé fault is the La Mesafault (Nichols, 1970). The trace of this fault may be followedfor 9 km from northeast of Pathé to the south of Pathé cliffwhere the unconsolidated clastic deposits of the San Antonio-Tecozautla valley conceal it. At the Pathé cliff, this faultstrikes rouhgly N-S and dips 80º to the west. A youngestbasalt flow shows about 40 m vertical displacement and about10° tilt against the fault plane.

Las Rosas fault. This fault strikes N15ºW and dips 80ºto the east, extending > 7 km northward from San Antoniohill. The trace of the fault is marked by a prominent scarp inpurple andesite lavas and white tuffs of the rhyolite unit. Onthe surface, the fault juxtaposes thick-bedded andesite lavasand tuffs against similar rocks capped by the pumice tuffunit (Figure 9).

Faults at the Mesa of Bomaxotha. South of the valleyof San Antonio-Tecozautla there is a sinuous cliff along theflows of ignimbrites and pyroclastics beds arising from theHuichapan caldera (Nichols, 1970, Silva, 1991, Milán et al.,1993) (Figure 3). On the surface, three straight high-anglenormal faults of kilometric length offset the bedding of thepumice tuff unit. This faultin–– g lies on the southwardextension of the Pathé and La Mesa faults. The easternmostfault has a vertical displacement of 30 m and 8 m of heave,inferred from a characteristic 3 m thick bed of blackish weldedtuff used as marker horizon. The fault plane strikes roughlyN-S and dips about 75º to the west. A discontinuous scarpmay be followed for at least 2 km. An about 0.70 m thickgouge, plus hydrothermal alteration, mark the fracture plane.

Just west of this fault there is another fault strikingN10ºW and dipping 75° toward the east. Light gray tuff bedsare displaced by about 20 m on the surface. On the top of theplateau there is a scarp with a height of about 1 m whichvanishes to the south. This fault and the previous one to theeast define a small graben 500 m wide and at least 5 kmlong.

Farther to the west, another normal fault west of thevillage of Bomaxotha displaces the beds of the pumice-tuffunit by 15 m. The fault plane strikes N10ºW and dips 80ºtoward the west. Together with the former fault they define ahorst 400 m wide.

DISCUSSION

Because of their direction and age the E-W and N-Strending faults are related to those existing in the northern

108


Fig. 7. View from the east of a normal faultoutcropping in Arroyo Bajhi, east of Rancho

Bajhi.

Fig. 8. View eastward of fault H at the TecozautlaRiver. The fault is nearly vertical and juxtaposesbasaltic lavas of the basalt unit (at right) against

beds of the pumitic unit.

Fig. 9. Northward, slightly oblique, view of thestrike of the Las Rosas fault at Rancho Las Rosas.The fault juxtaposes rocks of the rhyolitic unitagainst rocks of the pumitic unit capping andesite

lavas and felsic tuffs.

109


Mexican Volcanic Belt (Suter et al., 1991; Suter et al., 1992),and in the Basin-and-Range province of northern Mexico(Stewart, 1978; Suter, 1987b, 1991; Stockes and Hodges,1989; Henry and Aranda, 1992).

The Neogene normal faults are not related to thecompressional structures of the Paleogene of the Sierra MadreOriental fold-and-thrust belt. However, two balanced cross-sections across these areas suggested that the Aljibes half-graben decollement is at the same depth (~ -4 000 m bsl)than the basal decollement of the El Doctor thrust nappe(Suter, 1987a; Suter et al., 1995). They are related to eachother because both structures are detached at the same depth.

A further consideration which might relate the SierraMadre Oriental fold-and-thrust belt with the extension areais the origin of volcanism. The adjacent shortened section isvery probably thin-skinned (Suter, 1987a) with the basaldecollement leaning to the WSW, thus suggesting a“continental lithospheric subduction”. Rothé (1977) believedthat the crystalline basement of the detached Mesozoic covercould have been pulled down by the orogenic movementsand melted at 600ºC at about 20 km depth, even after theorogeny was over. Thus the granitic magma might haveerupted independently from the basaltic magma. The basaltsin this area as in most of the Central Mexican Volcanic Beltare younger than the rhyolitic volcanism (Schlaepfer, 1968;Nelson and Hegree, 1989, Cserna et al., 1987; Vázquez andJaimes, 1989; Mooser et al., 1997).Therefore they could haverisen to the surface during the deepening of the faulting. Thiswould explain the tendency of the volcanism to become morebasic in time.

CONCLUSIONS

The lithostratigraphy of the Pathé geothermal fieldconsists of Late Miocene to Pliocene volcanic rocks. Frombottom to top we find: the interstratified unit; the rhyoliteunit which includes basalts, andesites and principally rhyolitelavas and tuffs; the basalt unit (6.3 ± 0.2 m.y., 6.4 ± 0.2 m.y.),the pumice unit of Pliocene age; and fluvial and colluvialdeposits of Quaternary age. These rocks are deformed bymany near-E-W and N-S high-angle normal faults. Theroughly E-W striking faults labeled (E, F, G, H and I) are acontinuation to the south of the A, B, C and D faults of theAljibes half-graben(A, B, C, D)

The near-surface throw of most of the faults may beestimated as 20 to 30 m; but subsurface information showsthat the vertical displacement increases to 180 m below thesurface near the intersection of faults E and Pathé (Nichols,1970). Thus vertical displacement is larger at the center thanat either end, a feature which is also observed horizontallyalong the strike in the E, F, Pathé, and La Mesa faults.

The E-W set of structures consists of planar faultsdipping toward the south in the range of 65° to 90°. Three of

the southernmost faults face to the north and define smallgrabens. The overall geometry of this structure consists of 9major faults (A to I). All the E-W faults are planar, except Awhich shows the appearance of a listric fault dipping 45° atthe surface. The half-graben is characterized by a rolloveranticline. Within the rollover, hanging-wall layers are rotatedinto the detachment fault, and the amount of rotation increaseswith the amount of extension and at depth. The syntheticand antithetic faults of the southern area are interpreted asnear-surface collapse grabens formed by accommodation ofthe hanging-wall strain generated above the curveddetachment surface.

The Aljibes half-graben forms a tectonic depression atleast~ 18.5 km long and 12.5 km wide. This structure followsa roughly E-W elongated continuous basaltic outcrop.

The north-south trending Pathé, La Mesa and Las Rosasfaults define the San Francisco graben. The Pathé fault is theeasternmost master fault facing west, and the Las Rosas faultis the antithetic westerly one. This structure averages 12 kmlength and 8.5 km wide. The Pathé geothermal field lieswholly on the eastern margin of the San Francisco graben.Steam is found in the Pathé fault where it branches close tothe intersection with the E fault. Along the E-W faults,especially the E fault, there are various hot springs producingover 600 l/s of water with temperatures in excess of 30ºC.

ACKNOWLEDGMENTS

Field work and and discussions with M. Suter aregratefully acknowledged. Detailed criticism was obtainedfrom discussions with Barbara Martiny, Luis Delgado,Ricardo Riva Palacio, and reviewers. Discussions in the fieldwith Juan C. Moya, Andreas Kammer, Jaime Milán and J.Mojica were useful. Logistic and financial aid was given byProject CONACYT 0108P-T.

BIBLIOGRAPHY

AGUIRRE DIAZ, G. and F. McDOWELL, 1993. Nature andtiming of faulting and synextensional magmatism in thesouthern Basin and Range, central-eastern Durango,México. Geol. Soc. of Amer. Bull,105, 1435-1444.

CARRILLO, M. M., 1989. Estratigrafía y tectónica de la partecentrooriental del Estado de Querétaro. U.N.A.M. Inst.de Geol. Revista, 8, 2, 188-193.

DE CSERNA, Z., M. DE LA FUENTE, D., N. M.PALACIOS, L. TRIAY and S. L. M. MITRE, 1987.Estructura geológica, gravimetría, sismicidad y elacionesneotectónicas de la Cuenca de México. Bol. 104. Inst.Geología UNAM.

HENRY, C. D. and J. J. ARANDA G., 1992. The real southernBasin and Range; Mid-to late Cenozoic extension inMexico. Geology, 20, 701-704.

110


MILAN, M. and J. J. HERRERA, 1987. Aspectos geológicosimportantes de la exploración geotérmica del campo dePathé. Geotermia-Revista Mexicana de Geoenergía, 3,31-39.

MILAN, M., C. YAÑEZ, I. NAVARRO, S. P. VERMA andG. CARRASCO, 1993. Geología y geoquímica deelementos mayores de la Caldera de Huichapan, Hidalgo.México. Geofís. Int., 32, 261-276.

NELSON, S. A. and J. HEGRE, 1990. Volcán Las Navajas,a Pliocene-Pleistocene trachyte-peralcaline rhyolitevolcano in the northwestern Mexican Volcanic Belt. Bull.of Volc. 22, 163-197.

NICHOLS, C. R., 1970. The geology and geochemistry ofthe Pathé geothermal zone, Hidalgo, Mexico (Ph. D.thesis): Norman, Oklahoma, The University ofOklahoma, 178p. (Unpublished).

ROTHE, J. P., 1977. Séismes et volcans: Que sais-je? PressesUniversitaires de France, Paris, 126 p.

SCHLAEPFER, C. J., 1968. Resumen de la Geología de laHoja México, Distrito Federal, Estado de México y deMorelos. Carta Geológica de México. Instituto deGeología. Univ. Nal. Aut. de México.

SEGERSTROM, K., 1961. Geology of the Bernal-Jalpanarea, Estado de Querétaro, Mexico. U.S. Geol. SurveyBull., 1104-B, 19-85.

SEGERSTROM, K., 1962. Geology of south-central Hidalgoand north-eastern Mexico. U.S. Geol. Survey Bull., 1104-C, 87-162.

SILVA, M. L., 1991. Caldera de Huichapan o El Astillero,Univ. Nal. Aut. de México, Instituto de Geología. GuideBook. 18 p.

STEWART, J. A., 1978. Basin-range structure in westernNorth America. A review. In: Smith, R. B. and G. P.

Eaton, eds., Cenozoic tectonics and regional geophysicsof the western Cordillera: Boulder, Colorado, GeologicalSociety of America Memoir 152, 1-13.

STOCKES, J. M. and K. V. HODGES, 1989. Pre-Plioceneextension around the Gulf of California and the transferof Baja California to the Pacific Plate. Tectonics, 8, 99-115.

SUTER, M., 1987a. Structural traverse across the SierraMadre Oriental fold-thrust belt in east-central Mexico.Geol. Soc. of Am. Bull., 98, 249-264

SUTER, M., 1987b. Orientational data on the state of thestress in north-eastern Mexico as inferred from stress-induced borehole elongations. J. Geophys. Res., 92,2617-2626.

SUTER, M., M. CARRILLO M., M. LOPEZ and E.FARRAR, 1995. Active extension at the boundarybetween the trans-Mexican volcanic belt and the Basinand Range Province, Mexico. Geol. Soc. Am. Bull., 107,6, 627-641.

SUTER, M., M. CARRILLO M. and L. O. QUINTERO,1996. Macroseismic study of shallow earthquakes in thetrans-Mexican volcanic belt. Seism. Soc. Am. Bull. Inpress.

VAZQUEZ, S. E. and P. L. R. JAIMES, 1989. Geología dela Cuenca de México. Tópicos Geológicos de la Cuencade México. Sociedad Mexicana de Mecánica de Suelos,1-24.

_______________

Miguel Carrillo MartínezInstituto de Geología,Universidad Nacional Autónoma de México.Apdo. Postal 70-296, 04510 México, D. F.Fax: 550 66 44Email: [email protected]

111

normal faulting in the path© geothermal area, central mexico

Documents